Protection system



INVENTORI- JOSEPH J. GRINERICH BY.'-

ATToRNeY'; I

United States Patent O PROTECTION SYSTEM Joseph J. Griuerich, Milwaukee,Wis., assignor to General Electric Company, a corporation of New YorkApplication March 9, 1955, Serial N0. 493,101

6 Claims. (Cl. Z50-95) The present invention relates in general to thecontrol of penetrating rays, such as X-rays, and has more particularreference to a protective system for terminating the emission of raysfrom a ray source, at the conclusion of a safe ray emission interval, inthe event that associated interval timing means should fail to function.

While the invention is especially well adapted for application to thecontrol of penetrating ray sources comprising electrical power operatedX-ray generating tubes, it will be obvious that the invention is notnecessarily limited to the control of ray emission from power operatedtubes, but that its principles may be usefully applied to the control ofrays emitted from other sources, including bodies of ray emittingmaterial, under the control of selectively operable emission blockingmeans, such as electrical power operated shutter means.

An X-ray generating tube commonly comprises an anode and a cooperatingcathode enclosed in a sealed, evacuated envelope, such a tube operatingfor the production of X-rays as a result of impingement on the anode ofelectrons emitted by and at the cathode. The cathode may and commonlydoes comprise a filament adapted to emit electrons when electricallyenergized, as by the passage of electrical current therethrough.Electrons thus emitted by and at the filament may be impelled toward andcaused to impinge upon the anode at high velocity under the influence ofelectrical potential maintained between the anode and cathode.

During operation, the anode of an X-ray generating tube becomes heated,as the result of electron impingement thereon. As a consequence, theanode may operate at a temperature of the order of the melting point ofits constituent metal. Heat thus generated at the anode may bedissipated through the walls of the tube envelope, or by the circulationof a cooling iiuid in heat exchange relation with the anode, or by otherheat disspating means. Each individual ray generating tube, of course,displays its own characteristic heat dissipation rate, and Xraygenerating tubes are usually operated at selected energy levels suchthat the tube will not be dangerously overheated during the interval ofoperation at the selected input energy levels, the permissible operatingenergy level varying inversely with the duration ofthe requiredoperating time interval.

It is customary, therefore, n operating an X-ray tube, to adjust thelevel of tube energizing power in accordance with the required operatingtime interval, which, in turn, depends upon the particular use to whichthe generated rays are to be put. For example, X-ray equipment may berequired to produce rays during time intervals which may vary between ashort interval, say of the order of $430 of a second or less, and longerintervals of the order of several seconds, or even minutes, suchequipment being adjustable to appropriately vary the tube operatingenergy level in accordance with the selected operating time interval sothat the tube will not be injured or damaged when operated during theselected interval at the corresponding safe operating energy level. Tubeoperating energy levels 2,875,344 Patented Feb. 24, 1959 ICS employedfor short interval operation, however, if con tinuously applied to atube, during extended intervals of operation, will cause the temperatureof the tube anode to increase above the melting temperature of itsconstituent materials, thereby damaging or destroying the device, orotherwise rendering it unt for further service.

X-ray generating tubes may be employed as ray sources for variouspurposes, including diagnostic, therapeutic and graphic or picturingpurposes, requiring operation of the tube at various operating energylevels during intervals of unlike duration. To this end the tube may beemployed as a ray source, in conjunction with ray confining means, forapplying a ray beam as in an irradiation zone, forming a picturing ortherapy station. Among the several picturing procedures requiring theemployment of X-ray generating tubes, are the picture making proceduresaccomplished by means of spot film, Bucky diaphragm, cassette changer,and photoroentgeu equipment, the same forming picturing stations adaptedto be irradiated by rays emitted from a controlled emission source.

Spot lilm picturing equipment, as shown more particularly in U. S.Letters Patent No. 2,552,858 of May 15, 1951, on the invention of R. J.Mueller et al., in Serialographic Apparatus, comprises means forsupporting X-ray sensitive film in a suitable cassette, in normallyretracted position outwardly of a picturing zone, and for projecting thecassette successively to relatively offset exposure positions in thezone so that several adjacent portions of the film may be successivelyexposed to the action of X-rays in the picturing zone.

Bucky diaphragm picturing equipment, as shown more particularly in U. S.Letters Patent No. 2,115,755 of May 3, 1938, on the invention of J. B.Wantz et al., in Radiographic Diaphragm, comprises means for applying,on suitable X-ray sensitive film, rays emitted from a suitable sourceand delivered thence in succession through an object to be pictured anda Bucky diaphragm prior to impingement upon the film.

Cassette changer apparatus, as shown more particularly in U. S. LettersPatent No. 2,277,330 of March 24, 1942, on the invention of A. J. Kizaurin Plate Changer, comprises means for presenting a plurality of cassetteenclosed sheets or plates of X-ray sensitive lm in rapid succession, inposition for exposure to X-rays emitted from a suitable source.

Photoroentgen apparatus, as shown more particularly in U. S. LettersPatent No. 2,248,976 of July 15, 1941, on the invention of G. W. Filesin Radiographic Unit, comprises a screen of material adapted to becomeluminescent in response to the impingement of X-rays thereon, whereby ashadow picture of an examination object may be induced in the screen byapplying X-rays from a suitable source through the object and upon thescreen, the apparatus including camera means operable to produce anoptical record of a ray induced shadow picture on the screen.

As shown more particularly in U. S. Letters Patent No. 2,588,124 ofMarch 4, 1952, on the invention of A. J. Kizaur in X-ray Apparatus,Bucky diaphragm and spot film apparatus may both be mounted in a commontable structure, including a support panel upon which an object to beexamined, treated, or pictured may be disposed. As a consequence, eitherthe spot film or the Bucky diaphragm apparatus may be used at will. Asshown in said Letters Patent, the Bucky diaphragm apparatus is disposedbeneath or behind the support panel. It may be used in conjunction withan X-ray generator tube, forming an X-ray source, disposed above or infront of the support panel in position to project X-rays through thesupport panel, an examination object thereon, the Bucky diaphragm andthence upon ray sensitive film carried in the Bucky diaphragm apparatus.

The spot film apparatus in said Letters Patent No 2,588,124 is disposedabove or in front of the support panel. It may be used in conjunctionwith a ray source, such as an X-ray tube, disposed beneath or behind thesupport panel and mounted on suitable frame means which, in turn, issupported on the table structure. Such ray source may be used either inmaking pictures upon sensitive film supported in the spot iilmapparatus, or to induce shadow pictures in a lluorescent screen forminga part of said apparatus.

Obviously, separate ray sources may be employed for use in conjunctionwith the spot tilm and Bucky diaphragm apparatus of Letters Patent No.2,588,124, although, as disclosed in U. S. Letters Patent No. 2,208,258of July 16, 1940, on the invention of J. I. Grobe in X-ray Equipment, asingle X-ray source may be provided and positioned either above or infront of the support panel of the table for use in conjunction withBucky diaphragm apparatus, and alternately beneath or behind the supportpanel for use in conjunction with picturing equipment disposed above orin front of the support panel of the table.

It is thus common practice to utilize a single ray emitting source, notonly in conjunction with spot film and Bucky diaphragm apparatus of thesort disclosed in U. S. Letters Patent No. 2,588,124, but also to usethe same source in conjunction with photoroentgen and cassette changerapparatus of the sort shown, respectively, in U. S. Letters Patent No.2,248,976 and No. 2,277,330. Indeed, it is usual to employ a single rayemitting source at will, not only for the accomplishment of ray therapy,but also for tluoroscopic screen examinations, as well as for picturingpurposes in conjunction with spot lm, Bucky diaphragm, cassette changer,and photoroentgen equipment.

Where a single X-ray tube is so employed as a ray source, it isnecessary to accurately adjust the energy level at which the raygenerating tube is to be operated, as well as the duration of theoperating interval for each kind of service to be performed. The energylevel at which the ray generating tube is operated may, of course, bedetermined by adjustment of the electrical power supply system withwhich the generating tube is connected. The duration of the tubeoperating interval, however, is commonly determined by suitable timingapparatus, which may be controlled by sensitive ray detecting means, asdisclosed, for example, in U. S. Letters Patent No. 2,401,289 of May 28,1946, on the invention of R. H. Morgan et al., in X-ray Exposure TimingApparatus. Such ray detecting means may be disposed in the path of raysemitted by the source during the exposure interval, each separatepicturing apparatus having an associated detector. These detectors, inconjunction with suitable integrating equipment, determine the totalquanta of rays delivered during an exposure interval and serve toterminate the exposure interval when a predetermined ray quanta has beenapplied.

Where several different devices, such as spot film, Bucky diaphragm,cassette changer, and photoroentgen apparatus, are used at will, it isusual to employ a timing unit for terminating irradiation intervals asby disabling the associated X-ray tube power supply system, selectivelyoperable means being provided for connecting the timer with the raysensitive detector which is associated with the picturing apparatusselected for operation. In such an arrangement, however, it is possible,through operational error, to connect with the timing unit a detectorassociated with a picturing device other than that selected foroperation, in which case the timing unit will fail to function inresponse to ray emission from the ray source, thereby possiblysubjecting the ray generating tube to dangerous, if not destructive,overloading, while, at the same time, exposing the object being picturedto possibly dangerously excessive X-ray exposure.

An important object of the present invention is to provide means forterminating the application of penetrating rays in an irradiationstation, such as an X-ray therapy or picturing zone, after theexpiration of an irradiation interval beyond which continued energydelivery at the selected level may deleteriously affect the device.

Another important object is to provide means operable to prevent theoverloading of an electrically operated device adapted for operation,during intervals of limited duration, at adjusted safe operating energylevels, in the event of failure to terminate operation of the device atthe conclusion of a safe operating interval.

Another important object is the provision, in conjunction with an X-raygenerator tube and associated equipment, including timing means forterminating tube operation at the conclusion of a safe operatinginterval, at correspondingly safe operating energy levels, of protectionmeans, operable in the event of failure of the timing means, todiscontinue the supply of operating energy to the tube, after the samehas been in operation, at a selected operating energy level, long enoughto reach the safe limit of energy input to the tube.

Briey stated, the present invention may be applied in connection withapparatus, such as spot film, Bucky diaphragm, photoroentgen andcassette changer equipment, forming a plurality of irradiation stations,a penetrating ray source, such as an X-ray tube, adapted to apply raysat any selected one of said stations, and adjustable timing meansoperable to discontinue the application of rays at a so selectedstation, including ray sensitive detectors disposed each at acorresponding irradiation station and adapted to be controllinglyconnected selectively with the timer to actuate the same in accordancewith ray quanta applied at the selected station, as measured by theassociated detector at such selected station; and the objects of theinvention may be attained by providing auxiliary ray sensitive detectorelements, as of cadmium sulphide, each disposed at a correspondingstation in position to be exposed to rays applied to the timing detectorat said station, means being provided for controllingly connecting eachof said auxiliary detector elements with suitable disabling meansoperable to discontinue the application of rays at the selected stationwhen the timing detector at the selected station is controllinglyconnected with the timing means, the so connected auxiliary detector,when exposed to the action of X-rays, serving to hold said disablingmeans inactive.

The foregoing and numerous other important objects, advantages andinherent functions of the invention will become apparent when the sameis more fully understood from the following description, which, taken inconnection with the accompanying drawing, discloses a preferredembodiment of the invention.

To illustrate the invention, the single ligure comprising the drawingdiagrammatically discloses a plurality of picturing devices including aBucky diaphragm device 11, a spot lm device 12, a cassette changerdevice 13, and a photoroentgen device 14, said devices formingirradiation stations to which rays may be applied for picture makingpurposes. The illustrated equipment also includes suitable ray sourcemeans X adapted to emit rays and apply the same selectively in saidstations. While the invention is not necessarily limited to anyparticular kind of ray source, the same may comprise a single X-ray tubeX-l adapted selectively to be disposed in appropriate position toirradiate each of the several picturing devices. Alternately, the raysource means may comprise the generating tube X-l adapted for use inconjunction with certain of said picturing devices, such as the spotfilm device 12, the cassette changer device 13, and the photoroentgendevice 14, and an additional ray generating tube X-2 adapted for useonly in conjunction with the Bucky diaphragm apparatus 11. Thegenerating tubes X-l and X-2 may be of any suitable or preferredcharacter and may comprise each an anode 15, forming an electron target,a cathode 16 embodying a tilament adapted to be electrically energizedfor electron emission, and a sealed, evacuated envelope 17, hermeticallyenclosing the anode and cathode, means being provided for electricallyconnecting the anode and cathode with suitable sources of energizingpower disposed outwardly of the envelope. An X-ray generating tube maybe operated either as a self-rectifying device, by connecting the anodeand cathode thereof directly with a suitable source of alternatingcurrent power, such as the secondary winding of a transformer, or theX-ray tube may be operated by applying unidirectional power between itsanode and cathode, as by connecting a rectilier between the alternatingcurrent power source and the anode and cathode of the generating tube.

As shown, the generators X-l and X-2 may be energized from a primarysource of alternating current power S respectively through transformersT and T', said transformers having secondary windings 18 and 18',respectively connected between the anode and cathode elements of thegenerator tubes X-l and X-2. The primary windings 19 and 19' of thetransformers may be connected in parallel relation, one end of eachwinding being electrically connected with one side of the power sourceS. The opposite ends of the windings may be connected with the otherside of said power source through a selector switch 20 and the normallyopen switch contactors 2l of a relay switch unit 22. Accordingly, thesupply of operating power to the X-ray tube source means, and hence theapplication of rays at a selected irradiation station, is controlled bythe switch 22. It is, however, within the contemplation of the presentinvention to control the application of rays at an irradiation stationotherwise than by controlling the supply of actuating power to a raygenerating source, as, for example, by providing ray blocking means,such as a shutter, disposed between the source and the irraditionstation and normally biased to prevent delivery of rays from the sourceto the station, and means, such as a solenoid, for retracting theshutter from ray blocking position when the solenoid is energized, acontrol switch, such as the switch'21, being connected in circuit withsuch shutter retracting solenoid and a source of solenoid energizingpower, whereby to cause retraction of the shutter and consequentapplication of rays at an irradiation station when and so long as thecontrol switch is energized.

The relay unit 22 may include an actuating solenoid 23 adapted, whenelectrically energized, to cause closure of the contactors 21, therebysupplying energizing power from the source S to cause rays to bedelivered at the selected irradiation station. The relay actuatingsolenoid 23 may be connected in a control circuit C comprising asuitable source of energizing power S', a manually operable switch 24and a normally closed switch 25 of a relay switch unit 26. The switchunit 26 may include an actuating solenoid 27 adapted, when electricallyenergized, to open said normally closed switch 25. Since the switch 25is normally closed, closure of the switch 24 will cause application ofrays at an irradiation station as the result of closure of the switch21; and the application of rays at such station will continue until theswitch 25 is opened.

To control the opening of the switch 25, each of the picturing devices11, 12, 13, and 14 may include a timing detector D, which mayconveniently comprise a iluorescent layer 28 and an associated photocell29. The layer 28 may be disposed in the path of X-rays applied, forpicture making purposes, at the irradiation station in which the layeris mounted, said layer being disposed in position to receive rays afterthe same have passed through an object being exposed to picturing raysat the irradiation station, the layer 28 being preferably mountedimmediately behind the ray sensitive picturing film disposed at thestation. The layer of material 28 is adapted, when excited by rayimpingement thereon, to glow and emit visible light, the intensity ofwhich varies as a function of the intensity of incident X-rays, lightemitted by a layer 28 thus comprising a measure of ray quanta appliedthereto. In conjunction with the layer 28, each of the detectors Dassociated with the devices 11, 12, 13, and 14 comprises a photoelectriccell 29 disposed in position to receive visible light rays emitted bythe associated layer of uorescent material 28, the photoelectric cell 29being thus excited in proportion to X-ray quanta applied on theassociated layer 28. Accordingly, by connecting a cell 29 in circuitwith the solenoid 27, a suitable power source B, and timing apparatus 30of the sort disclosed, for example, in U. S. Letters Patent No.2,401,289, the solenoid 27 may be energized to open the switch 25 aftersource emitted rays have been applied at the selected irradiationstation during an interval measured by the timing apparatus 30 in termsof ray quanta impinging upon the layer 28 of the detector means D atsaid selected station.

Adjustable switch means 31 may be provided for selectively connectingthe several photocells 29 individually with the timing apparatus 30, thephotocells being also commonly connected with one side of the powersource B, the timer being connected with the other side of said powersource through the solenoid 27. By selective adjustment of the switch3l, any one of the ray detecting photocells 29 may be controllinglyconnected with the timing equipment 30. Accordingly, if rays from a raysource be directed upon the picturing device or irradiation stationcontaining the so connected cell 29, under the control of the relay 22and the switches 24 and 25, application of rays at said station may beterminated upon the opening of the switch 2S, at the conclusion of anoperating interval of duration determined by the timing apparatus 30,said apparatus being adjustable to measure any desired one of a numberof different operating intervals in terms of ray quanta applied at theirradiation station during the measured interval. If, however, rays fromthe ray source be directed, in error, elsewhere than upon a picturingdevice or zone containing the timer connected ray detecting photocell,the timing apparatus will, of course, fail to operate. As a consequence,the application of rays at the irradiation station may be continuedbeyond safe limits.

To obviate the dangerous conditions thus made possible by faultyadjustment of the switch 31, an auxiliary or safety detector 32 may bemounted at each of the several irradiation stations at which rays fromthe source means may be applied, said auxiliary detectors 32 beingmounted each adjacent the ray detecting photocell 29 at the station inwhich such auxiliary detector is mounted. As a consequence, eachauxiliary detector 32 will be exposed to rays from the source means onlywhen the photocell disposed at the same ray exposure station is alsosubjected to rays emitted from the same source.

While any suitable ray detecting means may be employed, the auxiliarydetectors 32 preferably comprise crystalline elements of cadmium ormercury sulphide, or cadmium selenide, the same having desirable X-raydetecting characteristics, as set forth in an application for U. S.Letters Patent, Serial No. 228,333, filed May 25, 1951, now Patent No.2,706,792 on the invention of John E. Jacobs in Semi-Conductors. Inaccordance with the present invention, the auxiliary detectors 32 areemployed to control disabling means for discontinuing the application ofrays at the selected station, in the event that rays emitted from thesource should fail to be applied upon the auxiliary detector associatedwith the timer connected photocell. To this end the circuit C, whichcontrols the application of source emitted rays at a selected radiationstation, in addition to the switches 24 and 25, and the relay coil 23,may include a normally closed, time delay opening switch 33 forming apart of the relay 22 and adapted to open and thus break the circuit C atthe conclusion of a preselected time interval following operation of therelay 22 in response to closure of the switch 24, such preselected timeinterval comprising a safe operating interval under alloperatingconditions.

Apparatus embodying the present invention also includes means operableto maintain the circuit C at the switch 33, after closure of the switch24, in the event that rays from the source be applied to the photocellcomponent of the selected detector D. As shown, such means mayconveniently comprise a relay 34 embodying a normally open switch 35,electrically connected in parallel with the switch 33, and a solenoid 36adapted to close the switch 35 when electrically energized. The solenoid36 may be connected in an electrical circuit including the power sourceB and a selector switch 31 connected with the several auxiliarydetectors 32 and operable in unison with the switch 31. As aconsequence, the switch 31' may be adjusted with the switch 31 toselectively connect the detectors 32 each respectively in circuit withthe solenoid 36 and the power source B when the photocell 29, which isassociated with the selected detector 32 at the same irradiationstation, is also connected with the timing apparatus 30 by means of theswitch 31.

In the event that the ray emission source means be disposed in positiondirecting its useful ray beam within an irradiation zone or stationother than the station containing the timer connected photocell 29, itwill be seen that the ray beam will also fail to impinge upon theauxiliary detector 32 which is controllingly connected with the relay34. The coil 36, accordingly, will remain de-energized and the switch 35will remain open. As a consequence, upon closure of the switch 24 toinitiate the application of rays at an irradiation station, the safetyswitch 33 will open after the expiration of a safe intervalcorresponding with the characteristic or set time delay opening intervalof the switch, and application of rays at the selected irradiationstation will be discontinued in response to the opening of the safetyswitch 33.

If, however, the ray source is oriented to apply rays at the irradiationstation containing a timer connected photocell 29, the beam will alsoimpinge upon the auxiliary detector 32 which is controllingly connectedwith the relay 34. The auxiliary detector, when so irradiated, willbecome conductive substantially instantaneously, thereby allowing thesolenoid 36 to become energized and close the switch 35. Closure of theswitch 35 will maintain the circuit C across the switch 33 when the sameopens after its characteristic time delay interval following closurel ofthe switch 24. As a consequence, the application of rays at the selectedirradiation station will continue until terminated in response to theopening of the switch 25 under the control of the timer equipment 30. ltwill be seen, thus, that the provision of the auxiliary detectors 32 ofnormally nonconducting semi-conductor material, selectively connectiblewith the relay 34 in unison with the connection of the correspondingphotocells 29 with the timing equipment 30, will allow properly timedapplication of rays at a selected irradiation station, in the event thatthe source means is correctly oriented to deliver rays at theirradiation station containing the timer connected photocell, butotherwise will operate to discontinue application of the ray beam withina safe interval.

lt is thought that the invention and its numerous attendant advantageswill be fully understood from the foregoing description, and it isobvious that numerous changes may be made in the form, construction, andarrangement of the several parts without departing from the spirit orscope of the invention, or sacrificing any of its attendant advantages,the form herein disclosed being a preferred embodiment for the purposeof illustrating the invention.

The invention is hereby claimed as follows:

l. A protection system, for use in conjunction with penetrating rayemitting means, comprising the combination, with operable means forcontrolling the delivery of rays from said emitting means to anirradiation station, of timer means controllingly associated with saidoperable means to actuate the same to terminate the delivery of rays tothe station, ray responsive detector means disposed at said irradiationstation and controllingly connectible with said timer means to actuatethe same, after the expiration of a safe exposure interval of minimumduration following actuation of said operable means to initiate raydelivery at said station, safety means operable to actuate said operablemeans for the discontinuation of ray delivery to the station, after theexpiration of a safe irradiation interval of Iminimum duration, andmeans for disabling said safety means in response to ray delivery atsaid station.

2. A protection system, for use in conjunction with penetrating rayemitting means, comprising the combination, with relay switch means forcontrolling the delivery of rays from said emitting means to anirradiation station, of timer means controllingly associated with saidrelay switch means to actuate the same to terminate the delivery of raysto the station, ray responsive detector means disposed at saidirradiation station and controllingly connectible with said timer meansto actuate the same, after the expiration of a safe exposure interval ofminimum duration following actuation of said relay switch means toinitiate ray delivery at said station, safety switch means operable toactuate said relay switch means for the discontinuation of ray deliveryto the station, after the expiration of a safe irradiation interval ofminimum duration, normally inactive relay means operable to disable saidsafety switch means, a ray sensitive detector disposed at said station,and means to controllingly connect said ray sensitive detector with saidnormally inactive relay means.

3. A protection system for use in connection with penetrating rayemitting means, comprising the combination, with operable means forcontrolling the delivery of rays from said emitting means to anirradiation station, of a timer controllingly associated with saidoperable means to actuate the same to terminate the delivery of rays tothe station, ray responsive detector means disposed at said station,means to connect said detector means with said timer to actuate thesame, a time delay opening switch connected with said operable means forterminating the delivery of rays from said emitting means to saidstation, after the expiration of a safe emission interval of minimumduration following actuation of said operable means to initiate rayemission, an auxiliary ray sensitive detector unit at said station, anda relay and means to connect same with said auxiliary detector unit,said relay including a normally open switch connected in parallelrelation with said time delay opening switch and closable in response toray mpingement on said auxiliary detector unit to render the time delayopening switch ineffective to terminate ray delivery at the station.

4. A protection system, for use in conjunction with penetrating rayemitting means adapted to deliver rays selectively to any one of aplurality of irradiation stations, comprising the combination, withoperable means for controlling the delivery of rays from said emittingmeans to a selected station, of a timer controllingly associated withsaid operable means to actuate the same to terminate the delivery ofrays to the selected station, ray responsive detectors disposed each ata corresponding one of said stations in position exposed' to stationdelivered rays, after the expiration of a safe exposure interval ofminimum duration following actuation of said operable means to initiateray delivery at said station, selectively operable means forcontrollingly connecting any selected one of said detectors with saidtimer to actuate the same, safety means operable to actuate saidoperable means for discontinuation of ray delivery at a selectedstation, after the expiration of a safe exposure interval of minimumduration, and means for disabling said safety means only ig the eventthat rays be delivered to the irradiation station in which the timerconnected detector is disposed.

5. A protection system, for use in conjunction with penetrating rayemitting means adapted to deliver rays selectively to any one of aplurality of irradiation stations, comprising the combination, withoperable means for controlling the delivery of rays from said emittingmeans to a selected station, of a timer controllingly associated withsaid operable means to actuate the same to terminate the delivery ofrays to the selected station, ray responsive detectors each mounted at acorresponding one of said stations in position exposed to stationdelivered rays, a selectively operable switch for controllinglyconnecting any selected one of said detectors with said timer, a timedelay switch connected with said operable means for terminating thedelivery of rays at a selected station, after the expiration of a safeexposure interval of minimum duration, a plurality of auxiliary raysensitive detector units, each disposed at a corresponding one of saidirradiation stations in position adjacent the station mounted rayresponsive detector, whereby both the ray responsive detector and theauxiliary detector unit may simultaneously be exposed to rays deliveredat the selected irradiation station, relay means including a normallyopen switch connected in parallel with the time delay switch andclosable to render the same ineffective to terminate ray delivery atsaid station, and means operable in unison with said selectivelyoperable means for selectively connecting said auxiliary detector unitscontrollingly with said disabling means when the corresponding rayresponsive detectors are controllingly connected with said timer.

6. A protection system, for use in conjunction with penetrating rayemitting means adapted to deliver rays selectively to any one of aplurality of irradiation stations, comprising the combination, withoperable means for controlling the delivery of rays from said emittingmeans to a selected station, of a timer controllingly associated withsaid operable means to actuate the same to terminate the delivery ofrays to the selected station, ray responsive detectors each mounted at acorresponding one of said stations in position exposed to stationdelivered rays, selectively operable means for controllingly connectingany selected one of said detectors with said timer, a time delay openingswitch connected with said operable means for terminating the deliveryof rays at a selected station, after the expiration of a safe exposureinterval of minimum duration, a plurality of auxiliary ray sensitivedetector units, each disposed at a corresponding one of said irradiationstations in position adjacent the station mounted ray responsivedetector, whereby both the ray responsive detector and the auxiliarydetector unit may simultaneously be exposed to rays delivered at theselected irradiation station, a relay having a normally open switch,connected in parallel with said time delay switch, and a coil operableto close said normally open switch, and means operable in unison withsaid selectively operable means for selectively connecting saidauxiliary detector units controllingly with said coil when thecorresponding ray responsive detectors are controllingly connected withsaid timer, to thereby energize the coil and render the time delayswitch ineffective to terminate ray delivery at the selected stationwhen the station mounted ray responsive detector is operativelyconnected with said timer.

References Cited in the tile of this patent UNITED STATES PATENTS2,401,289 Morgan et al. May 28, 1946 2,650,258 Pantchechnikoif Aug. 25,1953 2,706,792 Jacobs Apr. 19, 1955

